The invention is in the field of capillary gas chromatography and more particularly in the field of on-column injection gas chromatography using a column inlet.
The ultimate goal of gas chromatography would be to achieve baseline separation of any number of components in a mixture, to identify each component at any concentration, and to carry out the separation instantaneously. While this goal may never be realized, great strides are continually being made in improving separations, in increasing sensitivity, and reducing analysis time.
The introduction of capillary columns represented a major achievement in gas chromatography resulting in marked improvements in separability, sensitivity, and reduced analysis times. Capillary gas chromatography was further advanced by the innovation of the retention gap, a column inlet connected to an analytical column, having relatively low retentive power with respect to the active phase of the analytical column, usually being deactivated but uncoated with stationary phase. (See Grob's "On-Column Injection in Capillary Gas Chromatography", Huethig, N.Y. [1987], pg 583, herein incorporated by reference.) The retention gap has become a preferred means for injecting considerably larger volumes of sample into a capillary gas chromatographic column than previously thought possible, resulting in as much as a several hundred-fold improvement in the signal to noise ratio for the analysis of solutes in the sample at a given concentration. There is no suggestion in the literature that a column inlet with equal or higher retentive power with respect to the active phase of the analytical column could be used for large volume on-column injection and, in fact, Grob (supra, pg 363) teaches away from this practice.
The practice of large volume on-column injection involves introducing a large volume of sample comprising a solvent and a component of interest into a retention gap usually at a temperature close to the boiling point of the solvent at atmospheric pressure. It is believed that a thin layer of the sample forms on the walls of the retention gap as a result of the rapid passage of carrier gas through the system. The solvent elutes first onto the analytical column followed by the volatile analytes which form a concentrated band on the analytical column. This concentration of volatile analytes leads to correspondingly sharp peaks in the chromatogram.
While retention gap technology has enabled large sample volumes of many types of compounds to be successfully chromatographed, certain classes of compounds, namely compounds not mobile in the retention gap at temperatures near the boiling point of the solvent (hereinafter referred to as non-volatile compounds), are still difficult to chromatograph. Unlike the volatile analytes, these non-volatile compounds do not concentrate at the analytical column at temperatures close to the boiling point of the solvent but instead, tend to remain spread along the wall of the retention gap. Even upon ramping the temperature of the oven, the peaks corresponding to these compounds are often broad and difficult to integrate. The ability to concentrate these compounds at the front of the analytical column would clearly be advantageous and would result in an improvement in sensitivity as well as repeatability of integrated signal.